Miniaturized devices for conducting chemical and biochemical operations have gained widespread acceptance as a new standard for analytical and research purposes. Provided in a variety of sizes, shapes, and configurations, the efficiency of these devices has validated their use in numerous applications. For example, microfluidic lab chips are utilized as tools for conducting capillary electrophoresis and other chemical and biochemical analysis in a reproducible and effective manner. Microarrays or Bio-chips are used to conduct hybridization assays for sequencing and other nucleic acid analysis.
In a typical labchip, materials are electrokinetically driven through interconnected microchannels. Electrodes are positioned in reservoirs fluidly connected to the microchannels to make electrical contact with a medium contained therein. Application of a voltage across two electrodes will drive material from one reservoir to another based on electrokinetic transport phenomena. In a microfluidic device having numerous channels and reservoirs to perform multiplexed procedures, an electrode array (e.g., 10 to 100 or more electrodes) may be positioned such that each electrode makes electrical contact with the medium in the device. Programmable controllers may be electrically connected to the electrodes to individually drive the electrodes in a controlled manner. Examples of the use of voltages and electrodes to transport materials electrokinetically are disclosed in, for example, U.S. Pat. Nos. 5,126,022; 5,750,015; 5,858,187; 6,010,607; and 6,033,546.
Various problems arise, however, when electrodes are “dropped in” reservoirs on a chip. First, the electrodes are subject to contamination from previous testing. An electrode dropped into one test chip may introduce unwanted material into a device subsequently tested, thereby contaminating the subsequently tested chip.
Additionally, when conventional metal electrodes (i.e. platinum, gold, etc.) are used to apply electrical fields within certain conductive media such as aqueous conductive media, bubbles are prone to form thereby disrupting the intended operation of the device. This problem is exacerbated in applications such as capillary electrophoresis where higher voltages are desirable to achieve more efficient separations (i.e. higher throughput, better resolution, etc.).
Within an electrophoretic channel or in a reservoir connected thereto, gas bubbles (e.g., an air bubble) can interfere with the electrical connection or otherwise change electrical properties between driving electrodes and the conductive medium. When bubbles are formed, electrokinetic operations can be severely or completely inhibited. Accordingly, current protocols for conducting capillary electrophoresis utilize remedial electrode configurations and are limited to voltages that will not generate substantial bubbles. These conventional protocols, however, typically do not achieve the desirable higher throughput of systems employing relatively higher voltages.
“Dropped in” electrodes must also be carefully aligned and the depth must be controlled. Positioning the electrodes too deep may break the electrode or damage the device; positioning the electrode too shallow may prevent application of a voltage to the medium in the device and thus prevent driving the sample material through the device. Further, moving electrodes into position adds complexity to the instrument used to carry out the testing.
It is therefore desirable to provide a microfluidic device that does interfere with the intended operations of the microdevice yet can still be integrated with electrically conductive components necessary for chemical and biochemical operations, e.g., heating elements, electrodes, electrochemical detectors, valves, flow detectors and the like.